Bilayer Depth Dependence of Hydrophobic Amino Acid Transfer Free Energies

Abstract

Over 15,000 disease causing mutations have been identified for membrane proteins in humans, the majority of which are thought to disrupt folding or trafficking. At its core, membrane protein folding involves the transfer of amino acid side chains from an polar, aqueous solvent into a lipid bilayer. Previously, we measured the transfer free energies of side chains to the nonpolar center of a lipid bilayer in two membrane proteins (beta-barrels OmpLA and PagP). However, the polarity and solvation state of the lipid bilayer is heterogeneous and contingent upon the position across the bilayer normal. Thus, it is imperative to measure the bilayer position dependence of these energies for all twenty naturally occurring amino acids to accurately describe membrane protein folding. Presently, we have measured the transfer free energies for all hydrophobic amino acids at six different depths in the membrane using OmpLA as a protein scaffold. We find, surprisingly, that hydrophobic residues energetically prefer to be about halfway between the bilayer center and the lipid head groups. Using these results we have calculated a depth dependent function describing the nonpolar solvation parameter in the membrane. These results allow us to begin to better predict the consequences of mutations and conformational changes on membrane protein stabilities and structure in a depth dependent manner.